Brazing materials containing titanium (i.e., active element) are well known in the art. The titanium content of these materials is generally greater than 5% by weight. The brazing materials therefore exhibit a relatively high yield strength and upon brazing contain a brittle dispersed phase.
It is, however, well known in the art that the thermal expansion mismatch between many materials, particularly a metal and ceramic member, requires the use of a ductile brazing material. Illustrative are the gold-nickel-titanium brazing alloys disclosed in U.S. Pat. No. 4,938,922. However, in order to achieve the desired ductility the titanium content of these alloys is generally maintained at very low levels, i.e. 0.1% to 2.0% weight percent.
The gold-nickel-titanium alloys of the noted patent may be employed in a single step process which produces a ductile brazed joint with excellent oxidation resistance at 650.degree. C. and no visible reaction to acid and alkali treatment. The alloys are particularly suitable for brazing silicon nitride ceramic to Incolloy 909 alloy for use in internal combustion engines, and are currently produced and sold by The Morgan Crucible Company plc, Wesgo Division.
The gold-nickel-titanium alloys of the noted patent are also useful for brazing superalloys such as Incoriel 718 and other superalloys containing aluminum and/or titanium. Both aluminum and titanium are strong oxygen getters resulting in a thin layer of oxide upon heating such superalloys, making wetting with conventional alloys difficult. It has, however, been found that wetting can be accomplished by increasing the temperature until optimal wetting is obtained, but, to the detriment of prior brazes made at higher temperatures.
For example, a 50% gold-50% copper alloy with a liquidus temperature of 970.degree. C. will typically flow at about 1,000.degree. C. However, when brazing an Inconel 718 alloy, the brazing is generally performed at .about.1,070.degree. C. A copper member with a melting point of 1,083.degree. C. may thus tend to melt close to a furnace heating element since most commercial furnaces exhibit temperature fluctuations of about 10.degree.-25.degree. C. within the furnace area.
The gold-nickel-titanium alloys disclosed in U.S. Pat. No. 4,938,922 will exhibit sufficient wetting to an alumina ceramic, but, over a very narrow temperature range of about 20.degree. C. Exceeding this temperature results in dewetting--that is, the molten alloy beads tip leaving bare ceramic where molten brazing material formerly coated the surface.
It has, however, been found that if vanadium is substituted as the active metal in the brazing material, higher quantities of vanadium (compared to titanium) may be employed while still maintaining the desired ductility. The increased quantity of active metal, in this case vanadium, will also significantly enhance the wetting characteristics of the brazing material over a broader range of temperatures.
Illustrative are the ductile gold-based alloys described in principal but not exemplified in U.S. Pat. No. 4,606,978, containing gold, nickel, vanadium and, optionally, molybdenum to prevent high temperature creep. The molybdenum content for these alloys is, however, maintained at high levels, i.e., 6-40% by weight.
What has, however, recently been found is that the addition of very low levels of molybdenum to gold-nickel-vanadium brazing materials, such as those described in U.S. Pat. No. 4,606,978, will significantly enhance the ductility of the brazing material. The brazing material will therefore exhibit a low yield strength. The lower yield strength results in lower residual stress in a brazed joint since the plastic deformation of the brazing material accommodates the thermal expansion mismatch between articles being brazed. The brazing material will also be easier to mechanically reduce/deform (i.e., rolling processes), minimizing any edge cracking which is generally associated therewith.
It is therefore an object of the present invention to provide a brazing material that can be directly brazed to a substrate, particularly a metal, surface over a broader range of temperature and yield a highly ductile, oxidation and corrosion resistant brazed joint.